This invention relates to a zinc anode can for a battery, such can being formed into the shape of hollow cylinder with one end closed, and a manufacturing method therefor, and a manganese dry battery employing such zinc can, particularly to a technique enabling a zinc anode can of high performance and a manganese dry battery using the same without adding harmful substances such as mercury, cadmium and lead thereto. As employed herein, the term zinc anode can refers to a can-shaped member to contain an electrolyte, to form an anode of a battery, and formed of a zinc base alloy.
As is well known, a zinc anode can used for a manganese battery is manufactured through the following series of steps.
1 A light amount of appropriate metal, mentioned below, is added to zinc and is melted therewith to form a zinc base alloy. PA1 2 Such melted zinc base alloy is cast continuously to obtain a continuous band-shaped sheet. PA1 3 Such band-shaped sheet, which was case continuously, is successively hot rolled at a temperature within a range of 200.degree.-250.degree. C. to obtain a plate-shaped sheet of a predetermined thickness. PA1 4 The thus rolled plate-shaped sheet is punched to obtain pellets of predetermined shape and dimension, such as round or hexagonal discs. PA1 5 Each pellet is pressed in a die and pressed by a punch impulsively to form a hollow cylinder with one end closed. (Impact backward extrusion method) PA1 6 An open portion of the hollow cylinder is cut away to adjust the height of the thus formed cylindrical zinc anode can.
Taking a zinc anode can for a R20 type manganese battery as an example, the thickness of the plate may be arranged to be approximately 5.2 mm at the rolling step 3. Then, round pellets of 30 mm diameter are punched out at the punching step 4, and the pellets are formed into hollow cylinders with one end closed, each having an outer diameter of 31.4 mm and thickness of 0.5 mm at the forming step 5. The height of the cylindrical can may be adjusted to 53.5 mm at the cutting step 6.
If the workability of the material for rolling or deformation processing (ductility) is not sufficient during the continuous hot rolling step 3 and the forming step 5 by the impact backward extrusion method, the material may suffer from cracks, burrs and so on, and successive working of the material will be interrupted. It is a basic and essential factor that the cylindrical cans are able to be formed at a high yield without causing defects such as cracks. (This is referred to as workability).
Finished zinc anode cans are carried to a battery assembly line, where a cathode, a separator and electrolyte are installed into the can. Then, the can is sealed with a cathode terminal and a gasket fixed to an opening of the can. When mechanical durability of the zinc can is too low at this stage, the can may be deformed during or after the battery assembly operation, thus causing various defects. Therefore, the finished zinc cans are required to have a certain mechanical strength. However, improvement in mechanical strength instead may deteriorate the above plastic workability (ductility), and vice versa.
A zinc anode can usually contacts the electrolyte contained therein in assembled batteries. Therefore, the zinc can must have sufficient corrosion resistance against the electrolyte in order to prevent self-discharge during storage of the batteries.
As required above, a zinc anode can of a battery is required to have characteristics such as plastic workability, mechanical strength after being finished as a can and corrosion resistance against the electrolyte. These characteristics relate not only to the composition of the zinc base alloy, but also to such factors of the manufacturing process as melting temperature at the melting step 1, die temperature at the molding step 2, temperature and reduction ratio at the rolling step 3, temperature at the punching step 4 for preparing pellets, temperature and workability ratio at the can forming step 5. (These factors are referred to as process factors.) Temperatures as process factors are specifically controlled to be maintained within 200.degree.-250.degree. C.
For the purpose of improving the above mentioned characteristics such as workability, mechanical strength, corrosion resistance, existing manganese batteries employ an anode formed of a zinc base alloy including approximately 0.15 wt % lead and approximately 0.05 wt % cadmium. As is well known, however, under the technical policy of avoiding as much as possible the use of harmful substances in the components of batteries, mercury was rejected first and then usage of cadmium was abolished. In summary, technical innovation seeks to employ metal additives of significant effect of improving characteristics and at the same time not deteriorating battery performance. (For example, see Japanese Patent Laid-open Publication No. 61-273861, Japanese Patent Publication No. 4-30712, Japanese Patent Laid-open Publication No. 4-198441, etc.).
However, even if recent manganese batteries, the zinc anode cans still include approximately 0.4 wt % of lead. It is the next technical target to avoid addition of lead.
In view of the above mentioned circumstances, in contrast to the existing zinc anode can of good characteristics but containing 0.4 wt % of lead, pure zinc cans were experimentally made for comparison and evaluation.
Zinc cans were made from base metal of zinc with zinc purity of 99.9986 wt % through the above-mentioned manufacturing process without addition of other metals. Experimental production of the cans were repeatedly performed while the process factors (melting temperature at the melting step 1, die temperature at the molding step 2, temperature and reduction ratio at the rolling step 3, temperature at the punching step 4 for preparing pellets, temperature and workability ratio at the can forming step 5) were varied. Accordingly, experimental products of different process factors were made without departing from the essential requirement (plastic workability) that cans without defect can be manufactured efficiently. Mechanical strength as a finished can and corrosion resistance to the electrolyte were examined for each can under the conditions below and the results were compared to those of the conventional products. (Comparison tests were performed on zinc anode cans for R20 type manganese batteries.)
(a) Sample pieces of 20 mm square were taken from a central portion of side walls of the formed cans. Vickers' hardness (Hv) of the sample pieces was measured at five (5) points for each piece, then the average value of the measured values for ten sample pieces was obtained. The average value was considered to be an evaluation of mechanical strength. PA0 (b) For evaluation of corrosion resistance, sample pieces of 10 mm square obtained in the same manner were soaked in electrolyte for a certain period, then loss by corrosion was measured. The average value of the weight loss among ten (10) sample pieces was obtained. The electrolyte was a water solution of a pH of 4.7, consisting of ZnCl.sub.2 (26.4 wt %) and NH.sub.4 Cl (2.2 wt %). The sample pieces were left in the electrolyte at 45.degree. C. for twenty (20) days.
As a result of the tests, the greatest hardness of the experimental products of pure zinc was Hv37, while hardness of the conventional products to which lead was added was Hv45. The least loss by corrosion of the trial products of pure zinc was 9.5 mg/cm.sup.2. With respect to hardness, the experimental products of pure zinc were not significantly inferior to the conventional products. However, the experimental products were very poor with respect to loss by corrosion. This result will prove the significant effect of addition of lead.
Next, the same zinc anode cans for R20 type manganese batteries as the preceding example were produced with zinc base alloy comprising pure zinc and additionally a small amount of indium through the above-described process. Vickers' hardness and loss by corrosion were measured by the same methods (a) and (b). The results have shown that a trial product containing 0.0010 wt % of indium had a hardness of Hv39.5 and loss by corrosion of 8.02 mg/cm.sup.2. These results were better than those of the above pure zinc cans. However, the cans to which this degree of indium was added were evaluated to be significantly inferior to the existing products, specifically with respect to loss by corrosion.
In view of this result, products containing more indium of 0.0040 wt % and 0.0100 wt % were prepared experimentally. In such case, workability of rolling of the material, i.e. the zinc base alloy containing indium, was so lowered that normal rolling could not be performed at the hot rolling step 3 and the material was broken into pieces.
This invention was made based on the aforesaidknowledge, and the objective of the invention is to provide zinc anode cans having excellent corrosion resistance and mechanical strength without adding harmful substances from an environmental viewpoint such as lead, cadmium, mercury, a manufacturing method of the same, and manganese dry batteries employing the zinc cans produced thereby.